The government, which had already resumed weighing schoolchildren, signaled this week that the next stage could be formal warning letters to parents notifying them if their offspring get too plump. It has also been subsidizing fruit and vegetables for young schoolchildren, encouraging low-income women to breastfeed, and funding programs to get children to walk and cycle to school.

But that's just a start. Experts advise that multiple policies are needed to rebalance a lopsided dietary equation in which food intake heavily outweighs energy expended.

"You can't just say, 'Eat less and be more active,' in a world where it's impossible to be active because the roads are congested and you can't walk anywhere and the only food you can get cheaply is not very healthy and you're advertising it all the time to people," says Neville Rigby of the International Obesity Taskforce in London.

On the intake side, experts say work needs to be done on food content (particularly salt, sugar, and fat), portion sizes, nutritional awareness, the high cost of healthy produce, and the marketing of unhealthy foods to kids. On the energy side, it's not just about getting people to exercise, but revamping towns to encourage people to walk, cycle, or even take the stairs instead of escalators or elevators.

New study foresees $90 billion cost

The global obesity epidemic has provoked alarm for a decade, with an estimated 400 million people worldwide now classified as obese. The World Health Organization predicts that figure will rise to 700 million by 2015.

But two recent studies in Britain have brought home the scale of the crisis here. A Health Department report this week found that obesity was more prevalent in Britain than any other European country; 23 percent of the population are obese – a threefold increase since 1980. In response, the health secretary, Alan Johnson, warned that obesity could be a "potential crisis on the scale of climate change."

The second, the Foresight Report, a major review conducted by 250 experts, warned that 60 percent of men and 50 percent of women could be obese by 2050, based on current trends – a health burden that would cost the country more than $90 billion a year. It said weight gain had become the default because human biology had not adapted to modern life, with its labor-saving devices, motorized transport, and sedentary work.

"The undeniable fact is that the pace of the technological revolution has outstripped human evolution," said Professor Peter Kopelman, one of the leading report contributors.

Helping youths stuck in junk culture

Kath Sharman has focused on helping children understand food in the five years she has run an obesity program in the northern city of Sheffield. She teaches young people about healthy eating, food groups, and different kinds of produce.

"We show them how to make smoothies," she says. "We do portion sizes, which is very important because a lot of young people are just eating too much."

But there is an educational battle to be fought as well, she says. Young people are steeped in a culture of junk, from the ads on TV to their leisure milieu. "There is lots of temptation there like fast-food outlets. Leisure facilities have vending machines full of high-fat foods. You go to the cinema and you are surrounded by food," says Ms. Sharman.

Some campaigners want more rigorous efforts to move "bad food" from the sightlines of young people or else clearly label it for what it is. Some suggest taxing bad foods and using the money to subsidize fresh fruit and vegetables which may look expensive to the poor.

Some countries, such as Denmark, have banned obesogenic "trans fats" from foods altogether, though efforts to secure international standards for reduced sugar, fat, and salt content in food have yet to amount to anything. Many countries still favor self-regulation, which has proven impotent against the might of the trillion-dollar global food industry.

Other countries, from Sweden and Norway to Brazil and Canada, have moved to limit advertising of junk to children. Britain has only restricted advertisers from targeting children's programs – that's not good enough, according to Josh Bayly of the British Heart Foundation.

"Most kids watch adult programming anyway," he says, calling for a ban on the advertising of junk foods before 9 p.m. He expresses disappointment at the failure to get more children to eat school meals.

A multimillion-pound program kick-started by a celebrity chef, Jamie Oliver, has apparently faltered: meals got healthier, but the number of takers dwindled. "School lunches are definitely part of the answer, but it was entirely predictable that take-up would fall," says Mr. Bayly, adding that not enough was done to make children enthusiastic.

Making streets more walking-friendly

Some experts say that it's just as important to change the sedentary lifestyle, the mobility of man, the "walkability of streets," the way buildings, streets, towns are designed.

"People need to be active for an hour minimum every day to get some benefit," says Mr. Rigby. "But the evidence is that people will not be that active without opportunities to do so. The environment has to be conducive."

Some schools have tried to reward children with badges and stickers for walking to school. But faced with ever-busier roads, many parents would rather not take the risk.

As for adult exercise, Professor Julian Legrand, a former health adviser to Tony Blair, suggests that large companies should be required to provide an "exercise hour" for employees. And rather than making it voluntary, he suggests that everyone be automatically enrolled, but have the right to opt out, instead of vice versa. This idea, he says, could be used to help bring about the cultural shift needed to get people to make better choices.

"Research shows that if you're automatically enrolled into something you'll stay, but if you have to opt in to it you won't," he says. "The idea is that you can take an idea of this kind and apply it to the public health field."

Adult obesity

Rates as a percentage of the total population:

US 30.6

Britain 23.0

Slovakia 22.4

Greece 21.9

Australia 21.7

Hungary 18.8

Czech Republic 14.8

Canada 14.3

Spain 13.1

Germany 12.9

Finland 12.8

Turkey 12.0

Belgium 11.7

Netherlands 10.0

Sweden 9.7

France 9.4

Switzerland 7.7

Japan 3.2

Source: Health Profile of England 2007, with data from the World Health Organization's June 2007 Health For All Database.